The invention relates to a method for the purification of gasification gas obtained from a carbonaceous material, in which method a gas stream made up of gasification gas and oxygen or an oxygen-containing gas added to it is contacted with a solid catalyst. In addition, the invention relates to the use of a catalyst in the purification of gasification gas.
The principal components of a gas obtained by the gasification of a fuel are carbon monoxide, carbon dioxide, methane, hydrogen, water vapor and nitrogen. In addition, the gasification gas typically contains small amounts of ammonia formed from nitrogen compounds in the fuel, as well as tarry organic compounds. The presence of ammonia in the gasification gas constitutes a problem, since in the combustion of the gas it turns into nitrogen oxides (NOx) detrimental to the environment. Tarry impurities also constitute a drawback, for example, when the gas is used for the production of electricity by means of an engine or a turbine or for the production of synthesis gas for the synthesis of methanol. Thus there exists a need to purify the gasification gas of detrimental components before its combustion or other further use.
It is previously known to remove ammonia from gasification gas by the scrubbing of the gas. This, however, involves the drawback of the waste waters formed, and the cooling of the gas is also a disadvantage when the gas is fed from gasification directly to combustion. Ammonia has also been removed by selective catalytic oxidation of the gasification gas, the catalyst used being, for example, aluminum oxide [1,2]. It has been observed that aluminum oxide also purifies gasification gas of tarry components when their amount in the gas is small.
Known catalysts suitable for breaking down the tar present in gasification gas include nickel catalysts and dolomites having an operating temperature of 800-900° C. [3]. In these conditions, nickel catalysts also break down ammonia. However, in operation, a nickel catalyst is also sensitive to impurities in the gas, and thus its operating efficiency decreases.
Catalytic purification of gasification gas has not yet progressed to commercial applications; the purification reactors are as yet only at the experimental stage. The high reaction temperature required in tests carried out with nickel and dolomite catalysts has been achieved with partial combustion of the gasification gas [4]. The catalyst has been located in the fluidized layer in the solid bed in the reactor, and the gas has been heated in a combustion chamber or zone preceding it so that the transfer of heat from the gas to the fluidized layer has produced the reaction temperature required by the catalysis.
It is also known per se to use catalytic combustion for the heating of the reactor and the catalyst bed. One example of this technology is the steam reformation process of hydrocarbons, wherein the reformer has been caused to operate autothermally by means of an alternation of catalytic combustion and reformation steps in the same catalyst bed [5].